1. Field of the Invention
The present invention relates to a circuit inspection apparatus for an electric circuit formed on an array substrate which employs a multiple pixel structure and an electric circuit equivalent to the electric circuit, and a method of manufacturing a liquid crystal display which employs the circuit inspection method and the multiple pixel structure.
2. Description of the Related Art
As a liquid crystal display, an active matrix liquid crystal display using a TFT (Thin Film Transistor) as a switching device is known. In the active matrix liquid crystal display, a liquid crystal material is sealed between a TFT array substrate on which scan lines and data lines are arranged in the form of a matrix and thin film transistors are arranged at crossing points between both the lines and a counter substrate arranged spaced apart from the TFT array substrate with a predetermined interval, a voltage to be applied to the liquid crystal material is controlled by the thin film transistors, so that a display can be achieved by using the electro-optic effect of liquid crystal. The ON/OFF-operations of the thin film transistors are controlled by voltages applied by the scan lines and the data lines. The scan lines and the data lines are connected to drive circuits, respectively.
In consideration of a tendency to improve definition of a recent liquid crystal displays, the number of data lines and the number of scan lines increase with an increase in number of pixels, and the number of drive ICs tends to increase. Since this tendency causes an increase in manufacturing cost and a decrease in yield, a structure (to be referred to as a “multiple pixel structure” hereinafter) in which a voltage is applied to a pixel electrode group belonging to a plurality of columns by one data line to reduce the number of data lines and the number of drive ICs to be connected to the data lines is proposed.
FIG. 14 is an equivalent circuit diagram showing an example of a structure of a TFT array substrate constituting a liquid crystal display having the multiple pixel structure. As shown in FIG. 14, for example, a pixel electrode A1 is connected to a scan line Gn+1 and a scan line Gn+2 through a first thin film transistor M1 and a second thin film transistor M2. A display signal is supplied from a data line Dm to the pixel electrode A1. A pixel electrode B1 is connected to the scan line Gn+1 through a third thin film transistor M3. Similarly, a display signal is supplied from the data line Dm to the pixel electrode B1. The other pixel electrodes are connected to the same circuit structures to sequentially supply the pixel electrodes A1, B1, C1, and D1 and display signals from the same data line Dm, thereby displaying an image. By employing the structure, as shown in FIG. 14, the number of data lines can be reduced. Consequently, the number of drive ICs connected to the data lines can be reduced. For this reason, an advantage of being able to reduce manufacturing cost can be achieved (For example, see Japanese Patent Application Laid-Open Nos. 2002-196357 and 2003-330034).
In general liquid crystal display, gradation displays on each display pixel are performed depending on quantities of electrical charge accumulated in pixel electrodes respectively arranged on the display pixels. Therefore, from a viewpoint of keeping quality of display image characteristics, it is important that appropriate charges are supplied to the pixel electrodes depending on display gradations and that the supplied charges are held for a predetermined period of time. In an inspection, performed in manufacturing steps, of a circuit structure formed on an array substrate, an electric charge writing function and an electric charge holding function especially related to pixel electrodes are intensively inspected.
A conventional inspection method related to a circuit structure formed on an array substrate is as follows. In general, as in an image display, thin film transistors corresponding to pixel electrodes are turned on, and known electric charges are supplied by data lines through the ON thin film transistors. A predetermined period of time after, the thin film transistors are turned on again, and charges held in the pixel electrodes are output outside through the data lines. For example, a value of charges in supplying is compared with a value of charges in outputting to determine whether the circuit structure of each display pixel is good or not.
It is desired that an inspection for a circuit structure be performed to all display pixels. For this reason, in a conventional inspection method, after predetermined electric charges are supplied to the pixel electrodes as in image display, the scan lines are sequentially scanned as in the image display to extract electric charges, thereby determining whether the display pixels are good or not.
However, a liquid crystal display which employs the multiple pixel structure, a defective pixel cannot be easily specified when an inspection is performed to circuit structures formed on an array substrate. This problem will be described below in detail.
As also shown in FIG. 14, the liquid crystal display which employs the multiple pixel structure has a structure in which the pixel electrode A1 is electrically connected to the data line Dm when the first thin film transistor M1 and the second thin film transistor M2 are turned on. Therefore, when electric charges accumulated in the pixel electrode A1 are output outside, the scan line Gn+1 and the scan line Gn+2 in FIG. 14 must supply voltages (to be referred to as “drive voltages” hereinafter) required to drive thin film transistors to turn on the first thin film transistor M1 and the second thin film transistor M2.
On the other hand, the third thin film transistor M3 arranged for the pixel electrode B1, as also shown in FIG. 14, has a configuration in which a gate electrode is connected to the scan line Gn+1. Therefore, when the drive voltage is supplied by the scan line Gn+1, the third thin film transistor M3 is also controlled in an ON state. The moment the pixel electrode A1 and the data line Dm are electrically connected to each other, the pixel electrode B1 and the data line Dm are electrically connected to each other.
A thin film transistor corresponding to the pixel electrode D1 has a structure in which a gate electrode is electrically connected to the scan line Gn+2. Therefore, when the scan line Gn+2 supplies a drive voltage to output charges accumulated in the pixel electrode A1 outside through the data line Dm, at the same time, the pixel electrode D1 and the data line Dm are electrically connected to each other.
As described above, when the pixel electrode A1 and the data line Dm are electrically connected to each other to output the accumulated electric charges outside, the pixel electrode B1 and the pixel electrode D1 are also consequently electrically connected to the data line Dm. Therefore, when the liquid crystal display which employs the multiple pixel structure is to be inspected, not only electric charges accumulated in the pixel electrode A1 but also electric charges accumulated in the pixel electrodes B1 and D1 are output outside through the data line Dm, so that only a sum of electric charges accumulated in the plurality of pixel electrodes can be recognized. It is very difficult to accurately determine whether the circuit structure is good or not.
The present invention was made to solve the above-mentioned problems and has an object of realizing a technique which can accurately detect quantities of electric charges held in a plurality of electric holding electrodes such as pixel electrodes, like an electric circuit formed on an array substrate for a liquid crystal display employed a multiple pixel structure.